A rolling bearing (hereinafter sometimes referred to simply as a bearing) used, e.g., in motors of rolling stock is conventionally protected from electrolytic corrosion by providing an electrically insulating film on at least one of the members with which the outer race or the inner race is in contact, e.g., the housing or the rotating shaft, so as to prevent an electrical current leaked from the housing or the shaft from running between the rolling members and the races to cause electrolytic corrosion.
For example, JP-A-3-277818 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes an anti-electrolytic corrosion rolling bearing having an insulating film formed of a polyphenylene sulfide resin (hereinafter referred to as a PPS resin) containing glass fiber. JP-A-5-240255 discloses a rolling bearing having an insulating film formed of a PPS resin containing glass fiber and nonfibrous inorganic insulating filler, such as calcium carbonate. The insulating film used in the former bearing is designed to have excellent creep resistance by using a glass fiber-reinforced PPS resin thereby to obtain stable anti-electrolytic corrosion performance. The insulating resin material used in the latter is designed to improve creep resistance without deteriorating injection moldability by using a PPS resin containing not only glass fiber but a nonfibrous inorganic filler thereby to obtain satisfactory moldability as well as stable anti-electrolytic corrosion performance.
The insulating film used in an anti-electrolytic corrosion rolling bearing is required of high creep resistance for the following reason. The insulating film in formed on at least one of the inner race and the outer race that are fitted between a rotating shaft and a housing with a given interference. If the film has poor creep resistance, it will wear thin with time, failing to maintain the interference constant.
With the increasing demand for speeding-up of rolling stock, the amount of heat evolved in a bearing has been increasing. However, any of the materials used in conventional insulating layers has a small thermal conductivity and thereby a high heat insulation action. For example, a synthetic resin has a thermal conductivity of about 0.2 to 0.4 W/m.multidot.K. The thermal conductivity of glass fiber, added to a resin, never exceeds the range of from about 1 to 6 W/m.multidot.K. Therefore, where a conventional insulating film is used as a film coating a bearing, the heat evolved on revolution of the rolling members hardly escapes, resulting in an increase of the temperature of the bearing. For the reason, further improvement in heat dissipation properties of the bearing is desired.
A rolling bearing in a fan motor is attached with a loose fit or a tight fit with its outer race to the housing and the inner race to the rotating axis of the motor, respectively.
Because a preload is imposed to the bearing by pressing the side of the inner or outer race via a spacer, etc. for the purpose of increasing the rigidity of the bearing and preventing vibrations in the case of not only a tight fit but a loose fit, an electrical connection is made between the outer race and a bracket and between the inner race and the rotating axis.
Increasingly, fan motors have been controlled by means of an inverter capable of varying the number of revolutions to control the air flow. In this case, the motor noise due to switching can be reduced by setting the carrier frequency of the inverter at a high level. The recent improvements in semiconductor devices and circuits have made it feasible. Thus, the carrier frequency of an inverter has now been set at a high level. It follows that the axial voltage generated in the inverter-driven motor increases, which makes a potential difference between the rotating axis and the housing, i.e., between the inner and outer races. Am a result, the electrical current passes through the bearing via the rolling elements. Thus, the possibility of electrolytic corrosion on the inner and outer raceways and the rolling surface of the rolling elements has been increasing.
The following countermeasures have been taken against the possibility of electrolytic corrosion in inverter-driven fan motors.
1) If possible, an electrical brush is set in contact with the rotating axis to keep the axis and the housing at equal potentials thereby preventing a current flow between the inner and outer races. PA1 2) Grease that is made electrically conductive is applied to the bearing to maintain the inner and outer races at equal potentials thereby preventing a current flow between them. PA1 3) Grease comprising a base oil having an increased viscosity is used to thicken the oil film formed between the outer race and the rolling elements and between the inner race and the rolling elements thereby electrically insulating the inner and outer races from each other. PA1 4) Bearings having an insulating film of resins, e.g., polyphenylene sulfide (PPS), or ceramics on the outer peripheral surface or the side of the outer race have recently been proposed.
These conventional countermeasures against electrolytic corrosion have their several disadvantages as follows.
Since the free end of the rotating axis of a fan motor is enclosed in a housing, there is no room for an electrical brush. To the other end of the axis is fixed a fan made of an insulating material, such as synthetic resins. Therefore, it is often difficult to place an electrical brush in contact with the axis. In practice, it is impossible in many cases to set an electrical brush in a fan motor. Even when possible, powder of the brush due to wear is discharged from the motor into the air as dust. This will be a fatal defect where a fan motor is to be used in a clean room.
Grease is made electrically conductive by incorporating a good conductor, such as carbon particles. Therefore, a bearing containing such electrically conductive grease is to make more noise than in using ordinary grease. Besides, use of such a special kind of grease that is expensive leads to an increase of cost.
When grease having a high viscosity base oil is used, the bearing torque increases due to the high viscosity, which results in an increase of temperature. Further, if an alternating current of high frequency passes between the inner and outer races, such a small space as an oil film thickness would be insufficient to shut the current. Furthermore, the state of the oil film undergoes changes due to deterioration of the grease with time or due to infiltration of foreign matter into the grease. As a result, it may occur that conduction and insulation alternate to cause electrolytic corrosion. Therefore, this method cannot be a perfect countermeasure.
As other measure of preventing electrolytic corrosion, the formation of an insulating film composed of plastic or ceramic on the outer race of the bearing is known. A glass fiber-reinforced PPS film, which is an example of the plastic insulating films, has a thermal conductivity of no higher than about 0.2 to 0.4 W/m.multidot.k. Therefore, the heat generated on revolution of the bearing hardly escapes, causing the bearing temperature to elevate. In this case, improvement in heat dissipation properties of the rolling bearing is further demanded. In the case where the insulating film is composed of ceramics, after flame spraying the ceramics, the formed film is subjected to cutting to a desired outer diameter and then polished to further increase the precision. Therefore, the cost involved is high an compared with the use of the plastic film.
The formation of an insulating film composed of a resin such as PPS or ceramic on the outer perpherical surface and the sides of an outer race not only results in high cost, but also causes some problems that the insulating film which comes to an interposition between the outer race and a housing makes it difficult to fit the bearing into the housing, and the insulating film is liable to creep between the housing and the outer race.
Attention has been paid to the above-described problems associated with the conventional countermeasures against electrolytic corrosion of rolling bearings used in fan motors.